A. Field of the Invention
The present invention relates to magnetic recording media installed in various magnetic recording devices, and to a method of manufacturing the media.
B. Description of the Related Art
Recently, “a perpendicular magnetic recording system” has been applied in practical use in place of conventional “longitudinal magnetic recording system” for enhancing the magnetic recording density. The recording magnetization is perpendicular to the medium surface in the perpendicular magnetic recording system. A perpendicular magnetic recording medium (hereinafter also simply referred to as “a perpendicular medium”) is mainly composed of a magnetic recording layer of a hard magnetic material, an underlayer for perpendicularly orienting the recording magnetization in the magnetic recording layer, a protective layer for protecting the surface of the magnetic recording layer, and a backing layer of a soft magnetic material for concentrating a magnetic flux generated by a magnetic head used for recording onto the magnetic recording layer.
In both longitudinal and perpendicular systems currently, the materials of the magnetic recording layer assume “a granular structure” in which ferromagnetic grains are separated by a nonmagnetic component. Preferable specific materials are CoPtCr alloy, and this alloy with an additive of oxide, for example, CoPtCr—SiO2 (Japanese Unexamined Patent Application Publication No. 2005-190552, which corresponds to US 2005/0142387 and Japanese Unexamined Patent Application Publication No. 2004-227740).
The perpendicular magnetic recording system has such advantages that the magnetization is more stable when recording density becomes higher, resistance to thermal fluctuation is better than in a longitudinal system, and signals are stably maintained even in small recording bits. In a medium of the granular structure, the grain size is controlled by an underlayer, and one magnetic crystal grain grows on one underlayer crystal grain to reduce magnetic interaction between magnetic grains and enhance the recording density, as disclosed in Japanese Unexamined Patent Application Publication No. 2005-190552, which corresponds to US 2005/0142387, for example.
However, since one bit is made up of a plurality of grains, “transition noise” due to a zigzag configuration between the bits along the grains causes a significant problem of degradation in signal quality. This problem is common in both longitudinal and perpendicular systems, and is a critical factor that limits recording density in the perpendicular magnetic recording system.
As a means to overcome this problem, “patterned media” in which one dot (one grain) is one bit are considered promising for a recording medium of the next generation. Because one bit is composed of one dot, the problem of “transition noise” is eliminated. So, the patterned media are expected to achieve higher recording density.
Several techniques appeared recently for manufacturing the patterned media. A typical example utilizes techniques in semiconductor processing, in which a substrate or a magnetic film is worked out to a dofted configuration using a resist film, and through a planarizing process such as polishing, a magnetic recording medium is produced (Japanese Unexamined Patent Application Publication No. 2003-16623, for example). Another technique forms extremely small holes (nanoholes) and fills the holes with magnetic material to form magnetic dots (Japanese Unexamined Patent Application Publication No. 2002-175621, which corresponds to US 2002/0086185 and US 2005/0031905, for example).
All the patterned media having regularly arranged dots as mentioned above involve a serious problem in the manufacturing process thereof. The problem is how to form and arrange the dots in a simple process. In a disk substrate as practically employed at present, it is very difficult to regularly arrange the dots in circumferential and radial directions. It is possible to make a dot pattern using a semiconductor process as described above, however, it requires a tremendous processing time. Moreover, it is very difficult to form every fine dot having a dot size less than 30 nm, for example, with sufficient accuracy. Processing of a magnetic film raises a problem of degradation of magnetic property due to thermal damage. Formation of nanoholes at the present state of the art presents difficulty in compatibility between minimization and regular arrangement. In addition, the process is complicated because a planarizing treatment of the surface is needed after filling the holes with magnetic material.
As described thus far, the proposed techniques of dot formation in the patterned medium are complicated and involve various problems in manufacturing the patterned medium. In view of the foregoing, it is an object of the present invention to provide a magnetic recording medium having magnetic dots achieving high density recording, and to provide a method of manufacturing such a magnetic recording medium in a simple process, and the present invention is directed to overcoming or at least reducing the effects of one or more of the problems set forth above.